1. Field of the Invention
This invention relates to an image forming optical system using a concave spherical mirror, and more particularly to an image forming optical system for converting a spot object into an arcuate image or an arcuate object into a spot image.
2. Description of the Prior Art
In an optical apparatus, there are some cases where an arcuate image is required. For example, in the printer for making semiconductor circuits disclosed in U.S. Pat. No. 3,963,353, a thin arcuate light source image on a mask is required as the means for illuminating the mask.
Usually, when an arcuate light source image is to be obtained, the light source must be arcuate in shape. Where a heating resistor such as tungsten lamp or the like is used, an arcuate light source may be relatively easily obtained by making the heating resistor arcuate in shape. However, a light source using such a heating resistor, because of its generally low brightness and the difficulty with which ultraviolet light is obtained from such light source, is not suitable for use with a printer which uses ultraviolet rays as the printing light due to the characteristic of photosensitive material.
As a light source which is high in brightness and from which ultraviolet rays may be obtained, there are gas discharge tubes such as xenon or mercury lamps. To make such a gas discharge tube into an arcuate light source, the glass tube must be made into an arcuate form. The glass working required to make the glass tube into an arcuate form is very difficult and the resultant arcuate gas discharge tube becomes expensive. The light source is a consumption article and particularly, the gas discharge tube having a high brightness is short in service life and this leads to the disadvantage of high running cost. Therefore, to obtain an arcuate light source image of high brightness, a spot light source must be used and made into an arcuate form by an optical system. Although somewhat different in meaning, a method of obtaining an arcuate light beam is disclosed in Japanese Patent Publication No. 13477/1973. According to this method, a conical mirror is optically sectioned along two planes orthogonal to the conical line thereof to obtain a band-like conical mirror and a light beam having a certain point as the origin of deviation is caused to impinge on such band-like conical mirror. The locus of the light beam on the scanning surface becomes an arcuate form centered at the conical line. However, the circular arc formed by such band-like conical mirror becomes wide in radial direction. This is because the reflecting surface of the band-like conical mirror has no curvature in the plane containing the conical line of such mirror and this reflecting surface has no converging action.
U.S. Pat. No. 3,758,774 discloses an image forming optical system for converting a spot light source into a linear light source image by the use of a paraboloidal or parabolic mirror. However, this U.S. Patent does not refer to an optical system for converting a spot light source into an arcuate light source image.